The present invention relates to the removal of H.sub.2 S and recovery of free sulfur from feed gases containing H.sub.2 S. More specifically, it is concerned with a process for the selective oxidation of H.sub.2 S to elemental sulfur in gas streams which may also contain H.sub.2, CO or light hydrocarbons, said selective oxidation being conducted in the thermodynamically favorable temperature range of 250.degree.-450.degree. F. The process is especially useful in the desulfurization of Claus tail gas streams.
The removal of H.sub.2 S and recovery of sulfur from H.sub.2 S-containing gases has been of major importance to industry. Petroleum refiners and natural gas suppliers in particular are concerned with such processes because H.sub.2 S is present in many refinery gas streams and natural gases. Its presence in such gases is undesirable because of its noxious odor, toxicity, corrosive properties and, recently, because of its contribution to atmospheric pollution. As a result, numerous processes have been advanced to obviate the difficulties associated with the use or disposal of gases laden with H.sub.2 S by removing it and effecting a conversion to marketable free sulfur.
The most successful method employed on a commercial basis which recovers sulfur from H.sub.2 S-contaminated feed gases (especially sour natural gases and the like) is a process in which the H.sub.2 S is first absorbed from the feed gases in solvents such as alkanolamines. These solvents are then stripped to recover a gas comprising about 85% H.sub.2 S and 15% CO.sub.2 which is then processed for sulfur recovery in a Claus plant. Quite typically, this Claus process involves the combustion of a portion of the recovered gas to obtain sufficient SO.sub.2 to provide a 1:2 mole-ratio with H.sub.2 S when the incinerated gases are recombined with the remaining recovered gas. This mixture is processed through a series of two or three reactors containing a bauxite catalyst which effects the oxidation of H.sub.2 S according to the well known Claus reaction: EQU 2H.sub.2 S+SO.sub.2 .revreaction.3S+2H.sub.2 O
The sulfur produced in each reactor is condensed in sulfur condensers situated after each reactor, thus desulfurizing the recovered gas in successive stages. Although this process is used in many industries, the economical removal of H.sub.2 S from the original feed gas stream to the Claus plant is limited to about 94 to 97% due to equilibrium limitations imposed by Reaction (I).
Because of the costly multi-step absorption-oxidation operations inherent in the Claus type of purification process, and the increasingly stringent environmental control standards, it has become a matter of great concern to develop a more economical process for the direct, and more complete, conversion of H.sub.2 S present in feed gas streams to elemental sulfur. Ideally, such a process would utilize only air or oxygen as an oxidant (without the necessity for separate facilities to produce SO.sub.2), would be entirely catalytic and be performed essentially in the gaseous phase. Also, the process should treat the feed gas directly, thus eliminating the costly absorption step. Heretofore, no process of this nature has been a practical possibility. Attempts directly to oxidize H.sub.2 S with air according to: EQU 2H.sub.2 S+O.sub.2 .fwdarw.2S (vapor)+2H.sub.2 O (II)
necessarily also result in formation of some SO.sub.2 according to: EQU 2H.sub.2 S+3O.sub.2 .fwdarw.2SO.sub.2 +2H.sub.2 O (III)
or EQU S(vapor)+O.sub.2 .fwdarw.SO.sub.2 (IV)
The SO.sub.2 produced by Reaction (III) or (IV) then reacts with H.sub.2 S as in Reaction (I), and the final conversion of H.sub.2 S is thus still dependent to some extent upon the equilibrium limitations of Reaction (I). Also, light hydrocarbons, CO or H.sub.2, if present in the feed gas, are usually oxidized to CO.sub.2, COS and water vapor, the formation of the latter further reducing conversion as defined by the thermodynamics of Reaction (I). The end result is not only a loss of sulfur recovery but also a possible loss of fuel gases and the production of an incompletely purified product gas.
Temperatures are of extreme importance in these oxidation processes because, as shown in FIG. 1, the thermodynamics of Reaction (I) permits the highest conversion of H.sub.2 S to sulfur at relatively low temperatures of 250.degree. to 450.degree. F. At these temperatures, however, the oxidation reaction kinetics are poor and no prior art catalysts are known which can effectively operate at these low temperatures. Additionally, at these low temperatures the condensation of sulfur on the catalyst may cause reactor plugging and/or catalyst deactivation. Efforts to deal with this condensation problem, such as by the use of swing reactors so as to permit frequent catalyst regeneration, increase the costs of operation. Hence, while low temperature operation is desirable, it is not without difficulties.
The several attempts to produce a competitive alternative to the Claus process and to effect direct catalytic oxidation of H.sub.2 S in a feed gas with air or oxygen at low temperatures resulted in, at best, only marginal results. This was due in large measure to the difficulties just mentioned. The earliest methods, as disclosed in U.S. Pat. Nos. 1,922,872 and 2,298,641, sought to employ bauxite catalysts of the kind utilized in the Claus process, or variations of those catalysts, to catalyze the desired oxidation. Such catalysts were found to be useful at temperatures above 450.degree. F. if the feed gas contained relatively high concentrations (&gt;10%) of H.sub.2 S, but at low temperatures or with low feed concentrations of H.sub.2 S the reaction tended to be sluggish and resulted in poor conversions. Generally, at such low temperatures the typical bauxite catalyst was found to be ineffective and required a follow-up operation, such as the absorption technique disclosed in U.S. Pat. No. 2,355,147, completely to remove the H.sub.2 S. On a commercial basis, bauxite type catalysts alone have been used only at high temperatures between 800.degree. and 1100.degree. F., at which temperatures H.sub.2 and light hydrocarbon gases are readily oxidized. Thus, although the conversion of H.sub.2 S was more rapid due to the kinetics of Reaction (I) at such temperatures, the bauxite catalyst lost its selectivity for oxidizing H.sub.2 S and conversions were necessarily poorer due to the thermodynamics of Reaction (I), as shown in FIG. 1.
The use of other catalysts at the desirable low temperatures of 250.degree. to 450.degree. F. has only been of limited success. For instance, sodium aluminosilicate zeolite catalysts, as described in U.S. Pat. No. 2,971,824, have been reported to lose their effectiveness rapidly at below 450.degree. F. Other catalysts of an alkaline nature, such as the alkali metal sulfides disclosed in U.S. Pat. No. 2,559,325, and the combination of alkali metal and alkaline earth metal oxides disclosed in U.S. Pat. No. 2,760,848, have been reported to produce no SO.sub.2 in the product gas even in the presence of excess oxygen. Since the formation of SO.sub.2 is inevitable even under conditions wherein insufficient oxygen (for Reaction (II)) is available, it is surmised that the SO.sub.2 reacted with the alkali present in the catalyst by an acid-base reaction. This would account for the lack of SO.sub.2 in the effluent gases and would result not only in a loss of alkalinity in the catalyst but also in rapid loss of catalytic activity and conversion efficiency.
Thus far, it has been shown that the need for an all gaseous phase, low-temperature, catalytic process for the selective conversion (i.e., in the presence of H.sub.2, CO and light hydrocarbons) of H.sub.2 S in a feed gas to sulfur is both great and unsatisfied. In many situations, however, the feed gas to be treated contains, in addition to H.sub.2 S, other gaseous sulfur components such as CS.sub.2, COS, SO.sub.2, etc., which should also be removed. If the feed gas contains SO.sub.2 and H.sub.2 S in a molar ratio of SO.sub.2 /H.sub.2 S less than or equal to 0.5, then it is theoretically possible to remove both sulfur components by oxidation-reduction. The reason for this lies in the fact that when these two components exist in such ratios, one need only add sufficient oxygen to the feed gas to give a ratio therein of (O.sub.2 +SO.sub.2)/H.sub.2 S equal to 0.5, the stoichiometric ratio required for Reactions (I) and (II) for the production of sulfur. More often, however, these ratios do not exist or the feed gas contains other gaseous sulfur compounds, of which a sizeable proportion exists as CS.sub.2, COS and mercaptans. To remove these gases, the prior art has relied heavily on the desulfurization process taught by Beavon in Canadian Pat. No. 918,384. In this process the Claus tail gas is contacted with an alumina-based cobalt-molybdate catalyst at temperatures from about 300.degree. F. to about 1200.degree. F. in the presence of sufficient H.sub.2 to convert essentially all of the sulfur gases contained in the tail gas to H.sub.2 S. These hydrogenated Claus tail gases are then oxidized in any convenient manner to yield elemental sulfur. One such oxidizing method disclosed in the Beavon patent is the Claus process previously described, with its attendant limitations. Another is the so-called Stretford process described in U.S. Pat. No. 3,097,926 wherein a rather expensive liquid phase oxidation effects the conversion of H.sub.2 S to elemental sulfur through the employment of a catalytic solution of sodium vanadates. Other processes have utilized various adsorption-desorption or absorption-reaction techniques to produce similar results; but no all gas phase catalytic process has been commercially employed for this purpose.
It is apparent from the foregoing that an all gaseous phase, low temperature, direct catalytic process for the air oxidation of H.sub.2 S contained in a feed gas to elemental sulfur would be highly desirable. It could be used alone to remove H.sub.2 S (or both H.sub.2 S and SO.sub.2 in specified ratios) from a feed gas stream, or in combination with a Beavon hydrogenation process, completely to desulfurize a feed gas stream containing other gaseous sulfur components (or H.sub.2 S and SO.sub.2 in other ratios). Furthermore, any H.sub.2, CO, or light hydrocarbons present in the gases to be treated by the aforementioned catalytic process should not be oxidized to form COS, water, or CO.sub.2 because: (1) COS is itself a noxious contaminant; (2) the formation of water tends to inhibit the conversion of H.sub.2 S to sulfur; (3) the oxidation reactions producing these undesired product components are very exothermic and this in turn creates difficulties in maintaining the operating temperature in the thermodynamically desirable range of 250.degree. to 450.degree. F.; and ( 4) the production of these undesirable products consumes oxygen, thus competing with Reaction (II) for the available oxidant. Because of these, and other difficulties mentioned hereinbefore, the development of a commercial, all gaseous phase, low temperature, catalytic process has not yet become a reality.